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Sam Trull
Sam Trull

The Human Who Teaches Orphan Sloths How to Be Wild Animals

Sam Trull
Sam Trull

Sam Trull was never prouder of Kermie than when she watched him fight another sloth for the first time.

The two-toed sloth was hanging upside down from a tree branch when the scuffle began. Kermie—strawberry blonde, orphaned, hand-raised by Trull, still learning how to be wild—scrapped with the bigger, older, and wilder Diablo, who clearly wasn’t thrilled with this newcomer to his small patch of Costa Rican rainforest.

Unexpectedly, a sloth fight isn’t some sort of slow-motion martial art that's not actually violent, like capoiera or tai chi. Instead, it involves quite dangerous combatants armed with sharp, bacteria-riddled teeth and dagger-like curved claws on their hands moving at a not-unusual speed. Yes, sloths are slow, but they can move faster than you think.

As Trull watched, Kermie and Diablo hissed and snapped and scratched at each other. The fight didn’t last long. After a few minutes, Diablo backed down.

Kermie had won.

“He was the first baby I ever raised from a newborn,” she says. “To know that he can interact with other sloths he’s never met—and then fight them and win—is amazing.”

Trull runs the Sloth Institute Costa Rica, a small nonprofit organization—co-founded by Trull, a zoologist, and her business partner, Seda Sejud, in August 2014—that rescues, rehabilitates, and releases orphaned sloths into the wild. She is one of the handful of researchers studying the six sloth species that live in Central and South America, and her work with the creatures is documented in her new book Slothlove (inkshares). Filled with gorgeous photos and fascinating facts, Slothlove touches on Trull’s journey from primatologist to sloth-ologist. But its focus is the many sloths she’s tried to help.

Wildlife rescue centers aren’t uncommon in Costa Rica, and several handle sloths. But few currently attempt to release the creatures back into the wild. In the past, that’s often been a death sentence for the animals.

Trull is trying nevertheless. She’s faced with an extraordinarily difficult task: rescue and heal a biologically unique mammal whose physiology is only somewhat understood; nurture it without taming it; train it in the sloth skills it will need to survive in the wild, even though we know little about how sloth mothers teach their babies; and release it into the rainforest, where it may not know which plants to eat (because we only know some of them) or how it will interact with wild sloths (because we know even less about their social interactions).

That’s why Kermie’s fight was such a huge milestone for Trull. When she met him, he was a tiny week-old orphan, the only survivor of a set of twins. She fed, nurtured, played with, nuzzled, snuggled, trained, and then eventually let go of Kermie, who after several months in a large training compound has been largely living independently since October 2015. Somehow, through a mix of instinct and, Trull hopes, her training, the human-raised Kermie is managing not only to live wild, but—if his scuffle with Diablo is any indication—even to kick some ass.

Kermie the sloth hangs upside down

Trull arrived in Costa Rica in 2013 after spending nearly two decades working with primates both in the U.S. and abroad. With a master’s degree from the UK's Oxford Brookes in primate conservation, she spent most of that time at the Duke Lemur Center, the largest prosimian sanctuary in the world.

In 2007, Trull lost both her fiancé in a car crash and her father to bone marrow cancer; the two died just six months apart. She spent the next few years wandering between the U.S. and Africa, grief stricken, directionless.

Then she moved to Costa Rica. She found a job at a small wildlife rehabilitation clinic on the Pacific coast called Kids Saving the Rainforest, which opened in 1999 in Manuel Antonio thanks to the efforts of two 9-year-old girls (with a lot of help from their mothers). The lush region draws many tourists, both Tico (Costa Rican) and foreign, and many expats have settled there. Between the hotels, homes, roads, and infrastructure, habitat encroachment has taken a serious toll on the region’s wildlife. The clinic needed someone trained in wild animal care to look after the many animals people brought to the clinic—squirrel monkeys zapped by electrical wires, sloths mauled by dogs, a wide variety of animals hit by cars.

Trull had experience in the field, and she needed a new purpose. In healing wild animals, she might also heal herself.

Less than a month after she arrived at KSRT, Kermie was brought in.

a baby sloth

As tree dwellers, sloths cling to each other for both safety and comfort; and so Kermie clung to Trull. As she would with the others, she fed him milk through a nipple-capped syringe, wrapped him in blankets, cradled him against her all the time. She had no idea how to care for a newborn sloth, but she had to do something.

Monster, Elvis, Ellie, Newbie, and Chuck (named after her father) all followed. Soon Trull had a half-dozen orphaned sloths living in her apartment. They were a mix of two-toed and three-toed sloths belonging to one of the two sloth species that live in Costa Rica: the brown-throated three-toed sloth (Bradypus variegatus) and Hoffmann’s two-toed sloth (Choloepus hoffmanni).

They all needed basically the same thing: a mother. (Fathers have no role in rearing.) In the wild, up in the trees, a baby sloth spends about six months in gestation (Hoffmann's two-toed sloths for 11 months) and about the same amount of time after birth clinging to its mom. It’s clear this is a safety issue; a fall from a tree can be deadly. Even if a baby sloth manages to survive, it’s then largely helpless on the forest floor, unable to escape predators or survive on its own.

It’s also a temperature issue. As heterothermic creatures, sloths regulate their body temperature through their environment. Holding onto mom keeps a baby sloth warm. It may also keep its gut bacteria at the right temperature to digest its leaf-heavy diet.

But there’s more to it than that. Sloths are intense cuddlers. They must touch, at least early on in their lives. Whatever physical contact they don’t get from Trull, they get from each other, forming ongoing alliances and relationships. Chuck rides around on Monster. Elvis play-fights with Bruno. If no companions are around, a stuffed animal will do for comfort.

Two-toed sloths also—for lack of a better term—make out. Tongues are involved. “They ‘French kiss,’” says Trull. While kissing appears to be a sign of affection, it also likely fulfills a biological function—perhaps the exchange of bacteria and enzymes.

Mothers also teach survival skills. Diet must be part of it. But does a mom sloth teach her child how to hide, sleep in a tree, or move from branch to branch? Or is this all instinctual? We don’t know. Sloths are notoriously difficult to observe because they are masters of stealth.

Trull did what she could to keep them fed, play with them, and let them explore within a protected environment.


Sam Trull
Chuck snuggles.

Her devotion to the sloths soon caught the attention of the BBC, who profiled her work in a series called Nature’s Miracle Orphans. (It aired in the U.S. on Nature.) The clip below highlights her attempts to care for four-month-old Newbie, a three-toed sloth whose mother had been killed by a dog.

As she did with the other orphan sloths, Trull tried to replace the mom sloth Newbie had lost as best she could. She fetched Newbie the perfect guarumo leaf to snack. She positioned Newbie’s cuddle pillow in the perfect patch of doze-worthy afternoon sun. 

Then Newbie got pneumonia. Four months of twice-daily oxygen treatments and injections couldn’t save her. After she died, Trull cradled her body for three hours. Her experience with Newbie cemented her desire to focus on sloths.

About a year later, in October 2014, someone brought to KSTR a pregnant sloth who had a severe head injury after falling out of a tree. She was having seizures—slow seizures, in sloth fashion. And then she went into labor. It wasn’t productive. Trull started to worry both mother and baby would die.

Detecting health problems in sloths is difficult because they are also masters of deception. When hiding is your best defense, cloaking your vulnerabilities is an important survival mechanism. “That makes it really hard to care for them because you don’t know something’s wrong oftentimes until it’s too late,” Trull says. “So you have to guess. Sometimes I go on my gut reaction. I can’t explain why something’s wrong, but something’s wrong. If you try to tell that to a vet, they look at you like you’re crazy.”

In this case, the vet Trull consulted about the pregnant sloth didn’t look at her like she was crazy. Instead, she ordered an x-ray. It revealed that the baby was breech. The two decided on an unprecedented course: a sloth C-section. Fully documented by Trull’s ever-present camera, the surgery made headlines worldwide.

Unfortunately, mother and baby both died a week later. The mother’s necropsy was inconclusive. The baby’s cause of death was no clearer. They classified it as a failure to thrive.

It was around that time that Trull shifted her attention to the Sloth Institute, which she and Sejud had launched just a few months before. The focus is the three Rs of wild animal care—rescue, rehab, release—along with one additional R: research.

a sloth undergoing a c-section

a newborn sloth

The big idea was to keep these animals as wild as possible during their time in full-time human care so that they could hopefully thrive in the jungle on their own. The tricky part was that the very human Trull was going to have to train them to be wild. Trull reached out to other sloth researchers in Costa Rica and Colombia for advice and insights and pored over the relatively scant scientific literature on sloths.

Then, inspired by the “boot camp” at the Duke Lemur Center—a forested enclosure where lemurs practiced being lemurs before being released in Madagascar—she and Sejud built a 19-foot-by-19-foot-by-19-foot cage near the field site where they hoped to eventually release sloths. (Trull found housing nearby; her home is currently sloth free.) The sloths spend several months in the enclosure, which provides a protected slice of rainforest where they can work on survival skills like climbing, finding food, not falling, moving slowly, being very still, and sleeping.

When Trull thinks they’re ready, she gives them a “soft release”: “The doors open, and then they can come and go as they please until they feel comfortable enough to be on their own and they’re eating enough wild food on their own,” Trull says.

So far, just two sloths have had a soft release: Kermie and Ellie, another two-toed sloth. Both are doing well so far.

Their movements are tracked thanks to the VHF (“very high frequency”) collars they wear. Trull’s four research assistants observe and track Kermie and Ellie every night from 4 p.m. to 6 a.m. (Two-toed sloths are nocturnal.) The first time Kermie slept all day in the rainforest—the nocturnal sloth equivalent of a first sleepover party—was, like the fight with Diablo, a milestone.

They also spy on wild sloths to find out what they eat and how they behave. If the researchers note a wild sloth eating a particular kind of leaf, for instance, they’ll gather some of those leaves and introduce them to the orphan sloths’ diet. Just last week they got permission to collar their first wild sloth, whom the research assistants, all British, decided to call Percy. Because Percy is of similar age and size to Kermie, “he will give us a better idea of what Kermie should be doing,” Trull says.

Understandably, there has been a lot of trial and error during this process. Not every sloth has survived. They generally don’t do well in captivity. Some have succumbed to the injuries that landed them in Trull’s care in the first place. Others withered away for mysterious reasons; they may have had genetic conditions that led to their abandonment. The first iteration of the boot camp cage didn’t work so well; built far from the release site, it wasn’t snakeproof, and Kermie and Ellie’s companion there, Pelota, was fatally bitten by a venomous fer-de-lance pit viper. Her death was a heartbreaking loss—and bitter lesson—for Trull, who is as openly in love with these animals as she is fascinated with them as a scientist.

“It was so devastating,” she says. “But it was also obviously a big learning experience.”

Not being snakeproof wasn’t the only problem with the cage. It also enclosed a patch of jungle that was less than ideal. If the trees had been taller, for example, Pelota may have been able to climb higher than a fer-de-lance can go.

They built the second cage near the release site, where the trees are “better,” Trull says, and the rainforest the sloths are meant to call home after release is right outside the cage door.

a two-toed sloth

One of the main challenges facing sloth rescue, rehab, and release is that there is still so much we don’t know about sloth biology, ecology, reproduction, social structure, or intelligence. Much of their lives remains mysterious.

Here is what we do know. There are two families and six species. Two are two-toed sloths, and four are three-toed sloths. Calling them two- or three-toed is a misnomer; it’s their hands that are different, so it’s more accurate to call them two- or three-fingered. (For simplicity’s sake, we’re calling them “-toed” throughout this article.) All species have three toes. They can have more bones in their neck than a giraffe—and the number of vertebrae varies by individual.

a sloth hanging upside town

Two-toed sloths are substantially bigger than three-toed sloths (13 pounds versus 9 pounds on average) and they have a broader diet than the strictly herbivorous three-toed sloth, eating eggs, insects, small vertebrates, and even dirt. In Costa Rica, hibiscus flowers and cinnamon tree leaves are favorites.

Their teeth are unique among mammals, lacking both incisors and enamel, which leads to discoloration from the leaves they eat. Their mouths are riddled with bacteria, and the two-toed sloths have a particularly nasty bite. One sloth clamped down on Trull’s ring finger while she was feeding it, and it was only thanks to people nearby prying the sloth’s mouth open that she was able to free her finger. She had to take systemic antibiotics for weeks, and in the end she still lost the nail.

Sloths have the slowest metabolisms and least muscle mass of any mammal of their size (and yet “abs of steel,” Trull writes, allow them to spend much of their lives upside down). It takes a long time for them to digest food; leaves can take a month to process. And although they can be affectionate in captivity, the famous sloth “smile” is actually caused by a lack of expressive facial muscles. In fact, they stress easily. A good indication of a freaked-out sloth is large pupils.

A sloth may only poop weekly or even monthly, and three-toed sloths take a long and dangerous descent to the forest floor to defecate and urinate. When they do go, they can lose about one-third of their body weight. Three-toed sloths bury their waste using their stubby tail to dig a hole. In captivity, they may relieve themselves almost daily.

They are indeed slow. Their sluggish metabolism, combined with a leaf-rich diet—which, from a food energy perspective, isn’t very rich at all—keeps sloths in first gear almost all of the time. That can be good news when it comes to avoiding attention. Sloth movements are so slow, they fall beneath the detection threshold of most predators. But it can be bad news for a sloth that mistakes an electrical wire for a branch, as happens often enough in Costa Rica. If it grabs on, it’s likely to get severely sizzled before it can let go. Its muscles simply can’t respond fast enough before it gets injured.

Despite their lazy reputation, sloths may not sleep as much as we once thought. A recent study [PDF] of wild sloths in Panama that found they sleep on average 9.5 hours a day in the wild. (They can sleep as long as 16 hours a day in captivity.) Being on the alert for predators may keep them awake longer.

Their hair is uniquely structured. They grow algae both on and inside their hair. The benefit of this is unclear: It may help them to blend into the trees better. It may also have a nutritional benefit. While some research suggests sloths may eat the algae, Trull is doubtful. (“I’ve never seen a sloth lick its arm,” she notes.) She is more inclined to support another theory: that the hair acts like a straw sucking the algae close to the skin, where its nutrients are absorbed. (One study found that a type of algae that only lives on sloths is passed from mother to child.) Their hair can be home to a range of other creatures, including moths, beetles, fungi, and roaches.

Their hair sucks up scents, too, and will hold onto them for weeks. For this reason, anyone who works with sloths slated for return to the wild can’t wear perfume, lotions, or—hard to imagine in the rain forest—bug spray.

They can live a relatively long time for mammals of their size: anywhere from 10 to 50 years.


Despite their similarities, two-toed sloths and three-toed sloths are quite different. Two-toed sloths are more active, aggressive, and nocturnal. Three-toed sloths are less energetic and confrontational, and they’re mostly diurnal.

A clue to their differences lies in their evolutionary history. Remarkably, two-toed sloths and three-toed sloths aren’t closely related. They split from each other at least 40 million years ago, and perhaps as far back as 64 million years ago. While two-toed sloths are descended from giant ground sloths, three-toed sloths owe their genetic line to some arboreal creature.

Sloths are one of the most extreme examples of convergent evolution, when the same environmental pressures cause similar adaptations in different creatures, resulting in uncanny similarities despite a lack of common ancestry (at least recently). While convergent evolution is a long-known, fascinating, and yet quite common phenomenon in its own right—another example is the long, sticky tongue that arose separately among anteaters, armadillos, aardvarks, and pangolin—it's notable that the adaptations these different sloth families made over millions of years rendered them so similarly unique. Or uniquely similar. Among mammals, sloths are, well, weirdos. Tens of millions of years of natural selection has led the different families to be weird in similar—though not identical—ways.

“They’re just completely different animals. They probably shouldn’t have the same name,” says zoologist Becky Cliffe. A Ph.D. student at Swansea University who has been studying sloths in affiliation with the Sloth Sanctuary, another wildlife rehabilitation clinic in Costa Rica, Cliffe is looking into the ecology and genetics of sloths. Her main tool? A backpack. Specifically, a GPS-loaded animal backpack originally designed for penguins by biologist Rory Wilson, Cliffe’s Ph.D. supervisor. In the past six years, Cliffe has strapped these backpacks onto 15 three-toed sloths and 9 two-toed sloths. Some she followed for three years continuously. She has noted multiple differences between the two species in Costa Rica, which represent the two sloth lineages.

“They’re 64 million years apart,” Cliffe says. “It’s like calling an anteater a sloth. They’re not the same. We can’t really group them as the same animal when we’re talking about them scientifically, and much less when we’re talking about rehabilitation programs. I think they both have very different requirements. But I think it’s something we’re only just really beginning to understand fully.”

Here’s one thing all sloth species have in common: in the forest, they're really good at being sloths. Overall, they’re quite successful and common throughout Central and South America. Their uniqueness works in the trees. The problem is, it doesn’t work well outside the forest, which is constantly getting smaller thanks to human encroachment. 


Beyond their physiology, much of the mystery surrounding sloths has to do with their interactions: namely, how they interact with their environment and with each other.

Thanks to the training and now the tracking, Trull is gathering a lot of new data about how they navigate their environment. One key insight: in order to survive, a sloth must be a careful cartographer of its local forest.

“For sloths, one of the most important things for them to learn is to basically map out the forest in their minds and to learn where they’re going,” she says. “That’s their biggest obstacle.”

She compares them to monkeys. “They’re similarly sized, and they’re both mammals that are arboreal,” Trull says. “But monkeys can just bounce around from tree to tree—bing bing bing! It’s no problem. And they can also see their food from far away. They can be like, ‘Oh, I want to bounce over to that tree that’s 15 meters away because they have some really big yummy berries.’”

Sloths are incapable of such improvised exploration, Trull says. “They need to know that tree’s there, they need to know they can get there, they need to know they can cross every single route through the forest and that berries will be waiting for them, because they can’t waste energy climbing 15 meters through a puzzle maze of trees just to get there and have no food.”

Cliffe says sloths are “masters of energy savings.” Every movement happens at the same speed, from blinking to grasping. She recently studied the metabolic rate of wild sloths by injecting them with doubly labeled water (in which certain isotopes of hydrogen or oxygen are replaced to allow for easier tracking) to measure how much energy they used during a two-week period. Sloths don’t eat much, because it can take them 30 days to digest the leaves in their diet.

“The energy supply is so restricted, that they have to save energy or they’re not going to be able to get from this tree to the next tree where they know they can eat the leaves safely,” she says. “I like to say they’re on the edge of the ‘energy budget.’ If they spend too much energy doing one thing, they’re not just going to have anything left to compensate. I don’t know if there are many other mammals that live quite on the edge like that. But they’re doing it quite well. They’ve been around for about 64 million years, so they’ve got the balance just about right. And it’s happening in both kinds of sloths, they’re just doing it in different ways.”

That likely explains their strategy from getting from place to place, Trull says. Once they find a route between points, they take it again and again. This also explains why roads are so dangerous to sloths. They don’t improvise well.  

A wild sloth crossing the road

Observing their locomotion has been revealing. Sloths use their body weight and careful timing to move from branch to branch. They also follow a basic rule that all rock climbers (and ladder users) know: always maintain three points of contact. Sloths have three limbs on the next branch before they let go of the previous one, Trull says.

Two-toed sloths also appear to be nearsighted, so visual acuity isn’t much of a factor in their forest navigation. Unlike a monkey, which can spot a sweet treat in your hand from far away, a sloth can’t see very far. That means they’re likely unable to spot food from a distance. They can’t rely on their eyes to plot a course.

Caribbean sloth in tree

All of this has both Trull and Cliffe curious about the nature of sloth intelligence. “The fact that have to have these mental maps of the forest just to get around must show some level of memory ability,” Trull says. “Also, I think—and I don’t really have any evidence yet to back this up except my gut, but—they definitely get a bad rap for being stupid and lazy. Yes, they’re slow, but their slowness is a part of their genius.”

While Trull is still mulling over a study design to test out sloth intelligence, Cliffe has attempted to measure it. “It didn’t go well,” she admits. They placed a three-toed sloth in an outdoor tree maze. It didn’t move. At all. “We gave up in the end. When sloths aren’t sure what’s going on or where they are, they sit still. That’s their defense mechanism.”

Cliffe says, “I don’t think they’re intelligent in the way you think a monkey or a dog is intelligent, but they’re smart in their own way in their mental maps and their memory.” In her six years of tracking, she could predict which branch of which tree they’d be on during a given day. “But if you cut that tree down, they’d be stumped. I think they’re smart in the ways they need to be, but beyond that, there’s not much there.”


Whatever their baseline intelligence, to survive and reproduce, sloths need to have some sort of social intelligence. While sloths are classified as solitary creatures, it’s clear there’s a strong bond between mother and offspring, and the orphan sloths in Trull’s care rely on each other for companionship, play, and comfort. As we mentioned, two-toed sloths kiss, sometimes upon encountering each other.


In the wild, there’s also the issue of sharing the forest. Kermie and Diablo have now scuffled three times (Kermie won twice), but another wild male in the vicinity shows little tendency to mix it up. What this indicates about sloth social structure is unclear, Trull says—“We’re still so new in our research”—but she notes that one previous study found what may be an “alpha” sloth with a group of females.

While they haven’t observed that behavior, it’s something to keep in mind when planning to release sloths. “The ratio of sexes in the wild is probably pretty important,” she says. “We probably shouldn’t release too many sloths in one area, because they’re just going to end up fighting, eventually.”

Diablo has also shown interest in Ellie. If they decide to mate, we know the basics of how that might unfold. Like humans, a female sloth goes into estrus once a month for about a week. She’ll make a high-pitched scream to attract a male. After mating, he’ll guard her for three or four days, and then the two will go their separate ways. 

“In terms of everything else, we don’t really know much about it,” Cliffe says. “There’s very limited observation as to what’s going on.”

After mothers raise their babies, which takes about six months, they leave the territory to their child, and move on themselves. But their ranges aren’t big, males and females live near each other, and sloths live quite a long time; one 25-year-old sloth Cliffe has observed is still going into estrus. That means generations of sloths can overlap in specific regions. Which makes Cliffe wonder: “How are they not inbred? Or maybe they are really inbred. Maybe that’s why they’re so weird.”

Cliffe has hair samples of all the sloths she’s been tracking, and a genetic analysis is underway that will reveal their genetic diversity—or lack thereof. “It’ll show me who is related to whom, and who’s the father of which baby,” she says. “So we’re going to get a lot of information from it.”

Kermie and Ellie are soon to be joined in the wild by Monster and Piper, both three-toed sloths. They’ve been in the boot camp cage for months. They’re slated for soft release on May 1, and they too will be collared and tracked.

Monster, who came in at two weeks old and is now 2.5 years old, is “my slothy soulmate,” Trull says. “Releasing her will be very emotional. I’m so excited for her. She’s done amazingly. Obviously, through the whole process, you know that they’re wild animals. But it’s also very amazing and reassuring to see their instincts kick in with certain things. At least they’re coming at this with some knowledge, and I don’t have to teach them everything. But to see them learn everything I’ve taught them is very rewarding as well.”

Hopefully, Monster and Piper will fare as well as Kermie and Ellie have. Trull and her team will continue to monitor their lives, all the while preparing the next batch of sloths for release.

a sloth eating a hibiscus flower

In Slothlove, the animals look downy, soft, and clean. That’s not because Trull ever gave them a bath. Being hand-raised by a human, they simply spent less time in the wild. But now Kermie and Ellie are out and about, and they’re changing. They have little interest in Trull, which is exactly what she was hoping for. Their fur is growing algae. And their scent is different.

“As they become more wild, their smell has changed. They smell really good now,” Trull says. “Not that they smelled bad, but Ellie and Kermie smell distinctly different hanging out in the trees all the time than they did before that—which is interesting.”

Their new scent is kind of a subtle testament to the fact that they seem to be moving—slowly, of course—towards a fully wild life in the forest. “It’s probably just coming from being in the trees and whatever saps that are getting on them from sleeping and moving in the trees versus living in my living room or sleeping in bags,” Trull says. “But they smell like trees now.” 

All photos © Sam Trull in Slothlove

You can keep up with the Sloth Institute’s work at their website, as well as through Trull’s Tumblr and Twitter feeds. And if you’d like to support their work, you can donate here.

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Animals
25 Icy-Cool Facts About Polar Bears
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From starring in Coca-Cola ads to becoming the poster child for climate change, the polar bear is quite the high-profile species. Ursus maritimus is a fascinating animal that roams across the Arctic Circle through Norway, Russia, Canada, Greenland, and Alaska, and there's more to them than the adorable faces you see in children's books and advertisements. Here are 25 facts you should know about the polar bear:

1. THEY'RE THE LARGEST CARNIVORES ON LAND.

A polar bear walks across the snow at sunset.
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Polar bears can weigh more than 1300 pounds and span more than 8 feet, 6 inches from nose to tail, making them the largest carnivores to currently walk the Earth. (Though other bears can grow larger, like Alaska's 10-foot-long Kodiak bear, they're omnivorous, while polar bears prefer an all-meat diet.) The males far outweigh their female counterparts, who may only weigh between 330 and 650 pounds. In general, though, a bear's weight fluctuates significantly throughout the year, with some bears packing on 50 percent more body weight over the course of a successful hunting season, then losing it over the course of their long fasting months.

2. BUT TECHNICALLY, THEY'RE MARINE MAMMALS.

A shot from below of two polar bears swimming in clear blue water
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Because they spend so much of their lives on ice, rather than land, polar bears are the only bears to be considered marine mammals. They hunt, court, and mate out on the ice, spending many months of the year far from land.

3. THEY'RE HIGHER ON THE FOOD CHAIN THAN WE ARE.

A large polar bear opens its mouth in a roar.
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Human beings aren't as high on the global food chain as you might think. Polar bears don't have any natural predators, and their intensely carnivorous diet puts them at the top of the food chain with species like killer whales, according to researchers, while humans fall somewhere closer to the middle. Don't worry too much about getting eaten by one, though—a 2017 study found that during the past 144 years, there have only been 20 fatal polar bear attacks in all of the five countries that have polar bear populations. However, as food becomes more scarce for the bears, humans living in polar territory may soon face more risk from starving bears.

4. THEY'RE LONERS …

A polar bear walks across a large field of ice.
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Other than the two to three years a cub spends with its mother, polar bears are pretty much solitary creatures. Adults spend only a few days a year mating, then go on their own way, spreading out to hunt on their own. They rely on the scent left by the sweat glands on their paws to track other bears, using the smell to sense where potential mates might be headed, among other things.

5. … BUT ARE SOMETIMES WILLING TO SHARE.

A polar bear sleeps cuddled next to her cub.
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Polar bears can play nice with each other sometimes. On occasion, they will hang out together in large groups, especially if there's a big meal that multiple bears can take part in, like a whale carcass. When they do spend time together (in what's called a sleuth), male bears will play-fight with each other, wrestling and swatting at each other without doing any real harm. According to the documentary Polar Bears: Spy on the Ice, polar bears can recognize friends they've met before even if they go without seeing each other for many years.

6. THEY'RE PICKY EATERS.

A polar bear shares food with a cub.
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When food is plentiful, polar bears are very selective about what they eat. They hunt seals, but if there are plenty available to hunt, they won't eat their whole catch. Instead, they'll only eat the energy-rich blubber (up to 100 pounds at a time), leaving the rest of the carcass for other animals to scavenge. When hunting is good, their diet is made up of about 90 to 95 percent fat. When times are lean, though, they'll happily branch out, eating reindeer, rodents, eggs, seaweed, and anything else they can get their claws on. However, because their bodies are so much better at digesting fat than protein, researchers think that if Arctic ice continues to melt and polar bears become unable access the ice (with its blubber-rich seals), they won't be able to get enough calories on land to survive [PDF].

7. THEY SPEND A LOT OF TIME FASTING.

A polar bear sprawls out on its stomach.
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When they're not out on the ice scoping out seals, polar bears spend an incredible amount of time fasting. The female polar bears fast longer than any other mammal species—in Canada's Hudson Bay, pregnant polar bears can fast up to 240 days, or almost eight months. There's reason to think they'll be fasting even longer in the future as sea ice melts, leaving bears with fewer hunting opportunities and less time to accumulate the fat stores needed to get through the lean months. During the 1980s, non-pregnant polar bears spent 120 days fasting between hunting seasons, but researchers now think that the bears will have to go without food longer and longer, fasting for as much as 180 days at a time in the future.

8. THEY WILL TRAVEL FAR TO FIND DINNER.

Two polar bears walk through the snow.
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The average bear might travel across 100,000 square miles in its lifetime, and that number may be getting higher. In 2013, a bear searcher told the BBC that polar bears were spending 9 to 13 percent more time being active to make up for the fact that the ice they hunt on is drifting faster, leaving them walking on a "treadmill" just to stay within their territory. One bear tracked by the WWF traveled almost 2300 miles from Norway to Russia in less than a year. Due to receding ice, polar bears have to walk farther to find prey, wasting valuable energy. The energy they gain from eating a single ringed seal might not even make up for what they expend trying to find and catch it.

9. THEY CAN SWIM FOR DAYS.

A polar bear swims toward the camera.
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Polar bears are savvy swimmers, paddling at an average speed of 6 miles per hour. And it's a good thing: Due to all that melting ice, polar bears are putting their swimming skills to lengthy use. In 2011, a study reported that a tagged female polar bear swam a total of 426 miles in one nine-day stretch across the Beaufort Sea above Alaska, losing 22 percent of her body weight in the process. Another bear in the study swam for 12 days, though she at least stopped to take some breaks.

10. THEY GET HOT FAST.

A wet polar bear sticks its tongue out.
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You'd think with all that plunging in Arctic waters, polar bears might get chilly occasionally. But since they're built to withstand extreme cold on a regular basis, they actually have the opposite problem: They overheat very easily, and are more likely to die from the heat than the cold. Their two layers of fur and solid layer of body fat (up to 4.5 inches thick) keep their metabolic rate consistent when temperatures reach as low as -34° F. They can sprint up to 30 miles per hour if need be, but much like you wouldn't want to run a race in a heavy ski jacket, polar bears can't spend much time chasing after their prey lest they overheat—a bear's body temperature can rise to feverish temps if they move too fast. On land, they typically only walk at speeds of three miles an hour, and their main hunting technique involves staying very still for hours or days at a time, waiting for a seal to emerge from the ice to breathe.

11. THEY'VE BEEN GETTING IT ON WITH GRIZZLIES.

A polar bear sits with her two cubs.
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In addition to changing their travel patterns and dinner prospects, climate change is altering polar bears' love lives. As the ice-traversing bears are forced to spend more time on the tundra, their habitats are starting to overlap with those of grizzly bears. In some places, the two species are getting more comfortable with each other, with amorous results. In Alaska and western Canada, grizzlies and polar bears are doing more cross-breeding, creating hybrid offspring.

12. THEY GROW A LOT IN THEIR FIRST FEW MONTHS.

A polar bear cub sits on its mother's back.
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At birth, polar bears weigh anywhere from 16 to 24 ounces—about what a guinea pig does. As newborns, they're blind, toothless, and only about a foot long. But by the time they emerge from their den for the first time around four months later, they are substantially larger, weighing between 22 and 33 pounds. In addition to nursing, they'll begin eating solid food around that time, and by 8 months old, they'll weigh 100 pounds or more.

13. THEY HAVE HUGE FEET.

A polar bear swipes its paws in the water.
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In order to balance on ice, polar bears boast giant feet. Their paws can measure up to 12 inches in diameter, acting like snowshoes to spread out their weight on thin ice and deep snow. The bumpy papillae (like the ones on your tongue) on their footpads help grip the ice, keeping them from sliding around. They also have long, curved claws that can measure almost 4 inches—all the better to grab onto slippery seals.

14. UNLIKE OTHER BEARS, THEY DON'T HIBERNATE.

A polar bear leaps into the water.
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While black bears, grizzlies, and other bear species spend each winter denning, forgoing eating, drinking, moving, pooping, and peeing for months on end, polar bears stay active all winter. Polar bears don't need to sleep through the winter, though, because there's plenty of food available to them in the coldest months, when they take to the sea ice to hunt for seals. The only exception is during pregnancy, when a female polar bear digs herself a den and remains sealed inside, surviving off her stores of fat, until her cubs grow large enough to survive outdoors.

15. THEY LOVE TO NAP DURING SNOWSTORMS.

Two polar bears sleep covered in snow.
iStock

Polar bears may not hibernate, but they are happy to lay low when bad weather hits. During the winter, they dig themselves into shallow pits in the snow to protect themselves from wind, sometimes remaining there for days as the snow piles up on top of them like a warm blanket. Sometimes, they take a similar approach to staying cool, digging through the tundra down to the permafrost during the summer to keep from overheating.

16. THEY'RE VERY HARD TO TRACK.

A polar bear wears a tracking collar.
Paul J. Richards, AFP/Getty Images

Considering how far they travel—both walking and swimming—over a given year, you can imagine how hard it is for scientists to track polar bears. By nature, they spend a huge amount of time alone in remote locations. Scientists use boats, helicopters, and low-flying planes to observe them, but that only works in good weather and in certain locations. So recently, they've turned to satellites, fitting bears with non-invasive radio collars and tracking them through high-resolution satellite imagery. It's cheaper than sending out a helicopter, and it lets researchers identify bears even in the most remote areas of the Arctic.

17. THEIR NOSTRILS CLOSE WHILE THEY SWIM.

A swimming polar bear peeks its nose out of the water.
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Polar bears don't have to worry about getting water up their nose. When they swim, their nostrils close to prevent them from breathing in water. They can swim at depths up to 15 feet, and while they typically only dive for a few seconds, they can hold their breath for more than two minutes, enabling them to sneak up on seals resting on ice floes. In 2015, scientists reported observing a record-breaking polar bear dive that totaled 3 minutes and 10 seconds. The hungry bear stalked three seals from afar, swimming almost 150 feet underwater without surfacing for a breath or to reorient himself to the seals' location before bursting out of the water where one of the seals was resting. (Sadly, his prey got away.)

18. THEY CAN TURN GREEN IN CAPTIVITY.

A polar bear in a zoo swims with a ball.
Ina Fassbender, AFP/Getty Images

Though polar bears are sometimes known as the white bear, they aren't white. Their hair is colorless and hollow, and only appears white because of the way light scatters through their fur. (Under that mass of hair, their skin is as black as their noses.) When bears are subject to warmer temperatures in captivity, though, they can take on a bit of a verdant hue. Algae infestations can turn polar bears green, and not just on the outer layer of their fur. The colorful algae grows inside the hollow tube of each hair. This green growth thrives in humid climates, like Singapore, where the bears don't naturally live.

19. THEY'LL NEVER MEET A PENGUIN.

A Bulgarian stamp set featuring a polar bear, a seal, penguins, and a walrus.
State Agency for Information Technology and Communications of Bulgaria, Wikimedia Commons // CC BY 2.5

Though you might see polar bears and penguins together in Coca-Cola ads or on winter-themed pajamas, the two species never mix in real life. They live at opposite ends of the Earth, though they both spend their days in icy waters. Polar bears exclusively inhabit the Arctic, and penguins only live in the Southern Hemisphere. The closest they ever get is when they live in the same zoo.

20. AT ONE ZOO, THEY POOP GLITTER.

Gold glitter on a black background
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At the Assiniboine Park Zoo in Winnipeg, Canada, the polar bears have sparkly poop. In 2014, zookeepers began feeding each of their bears a different color of non-toxic glitter so that they could trace their bowel movements, analyzing the samples to identify health issues, track stress hormones, and generally see how the bears are dealing with zoo life. The colors help the zookeepers label which poop comes from which bear.

21. EUROPEANS HAVE KEPT THEM IN CAPTIVITY SINCE THE 13TH CENTURY.

A 1938, black-and-white photo of a polar bear lying on its back in a zoo
Topical Press Agency/Getty Images

Captive polar bears have piqued public curiosity since as early as the Middle Ages, when the bears were occasionally given to European royalty by Viking traders. In the 1200s, when Henry III kept one in London, it was muzzled and chained but allowed to catch fish and swim in the Thames River. In the 17th century, Frederick I of Prussia kept a defanged and declawed polar bear, staging public fights between it and other large mammals for public amusement.

22. POSING WITH THEM WAS ONCE A POPULAR GERMAN PASTIME.

The orange cover of 'Teddybär' shows a person in a polar bear suit with his arms wrapped around a smiling boy.
Innocences

In the early 20th century, getting a picture with a man dressed in a polar bear suit was a fairly standard activity in Germany, at least according to the many photos found by French photo collector Jeann-Marie Donat. Donat spent 20 years tracking down the vintage photos, taken between 1920 and 1960, for his 2016 book Teddybär. There are several potential explanations for why so many Germans elected to stop for photos with people in polar bear suits (or to dress up as polar bears themselves). Donat suggests that it might trace back to the popularity of the two polar bears that arrived at the Berlin Zoo in the 1920s, while Hyperallergic notes that the costume was created as a Fanta advertising stunt, designed to distract Germans from the horrors of World War II. The photos show people young and old posing next to bears at the beach, in parks, in the street, in the summer and winter, alone and in groups. They all look delighted to get a chance at a polar-bear souvenir.

23. THEY CAN BE … POLARIZING.

Knut and his handler pose for photos lying down on their bellies.
John Macdougall, AFP/Getty Images

Knut, a polar bear cub born at the Berlin Zoological Garden in 2006, was hand-raised by zookeepers after being abandoned by his mother at birth. The cute cub became an instant tourist attraction—the most famous bear in the world, even—and the zoo's attendance rates skyrocketed, netting an extra $1.35 million in tickets when the bear began making twice-a-day public appearances.

But not everyone was psyched about "Knutmania." The young bear's popularity proved to be controversial for animal rights organizations like PETA, whose German spokesperson Frank Albrecht said the zoo should have let the orphaned Knut die rather than continue hand-feeding him, a process that he called a "gross violation of animal protection laws." In 2007, the bear received an anonymous, handwritten death threat from a hater who simply wrote "Knut is dead! Thursday midday." The zoo took the fax seriously enough to assign triple the amount of zookeepers keeping watch over the polar bear during his daily public romp. (Knut continued to live at the Berlin zoo until his death at age 4 from an autoimmune disease.)

24. THEY SOMETIMES GET THE CELEBRITY TREATMENT.

Photographers crowd in front of a barrier to photograph Knut at a zoo.
John Macdougall, AFP/Getty Images

In 2007, celebrity photographer Annie Leibovitz photographed Knut for the cover of Vanity Fair's annual "Green" issue. While Knut appeared solo on the cover of the German edition, he was Photoshopped into an image with Leonardo DiCaprio for the American edition. After his death, the Berlin zoo erected a bronze statue in his honor, and his body was preserved for display at the city’s natural history museum.

25. CHURCHILL, CANADA HAS A UNIQUE WAY OF LIVING WITH THEM.

A green sign in a snowy field reads 'Polar Bear Alert: Stop.'
Paul J. Richards, AFP/Getty Images

Churchill, a town in Manitoba, Canada on the shores of the Hudson Bay, is known as the polar bear capital of the world. During the fall, hundreds of polar bears pass through on their way to their icy hunting grounds on the bay, waiting nearby as the ice hardens for the winter. The locals have adopted unique ways of living with the hungry bears. Many don't lock their doors, so that if someone is running away from a polar bear, they can duck into any doorway. Since Halloween falls right in the middle of polar bear season in town, city employees, police officers, volunteer fire officials, and polar bear conservationists stay on patrol to drive away any bears that might be tempted to go trick-or-treating themselves, using helicopters, sirens, air horns, rubber bullets, and more to keep the bears at bay. Kids, for their part, aren't allowed to wear anything white for the evening.

Churchill also runs a "polar bear jail" for bears that continue to wander into town. Residents are encouraged to call the Polar Bear Alert Program hotline year-round if they see a bear in town, and conservation officers will come and try to scare it away. If shooting loud scare rounds at the bear doesn't do the trick, they trap the bear, or, if all else fails, hit it with a tranquilizer dart and take it to the Polar Bear Holding Facility. The specially-designed compound can hold up to 30 bears and is meant to keep bears that are aggressive or persistently return to the community. When the bay freezes, these bears are transported by helicopter or vehicle onto the ice, where they resume their normal winter hunting routine. With warmer temperatures keeping bears off the ice for longer and longer periods, more towns may soon have to learn from Churchill's strategies for peaceful coexistence.

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Animals
Where Do Birds Get Their Songs?
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Birds display some of the most impressive vocal abilities in the animal kingdom. They can be heard across great distances, mimic human speech, and even sing using distinct dialects and syntax. The most complex songs take some practice to learn, but as TED-Ed explains, the urge to sing is woven into songbirds' DNA.

Like humans, baby birds learn to communicate from their parents. Adult zebra finches will even speak in the equivalent of "baby talk" when teaching chicks their songs. After hearing the same expressions repeated so many times and trying them out firsthand, the offspring are able to use the same songs as adults.

But nurture isn't the only factor driving this behavior. Even when they grow up without any parents teaching them how to vocalize, birds will start singing on their own. These innate songs are less refined than the ones that are taught, but when they're passed down through multiple generations and shaped over time, they start to sound similar to the learned songs sung by other members of their species.

This suggests that the drive to sing as well as the specific structures of the songs themselves have been ingrained in the animals' genetic code by evolution. You can watch the full story from TED-Ed below, then head over here for a sample of the diverse songs produced by birds.

[h/t TED-Ed]

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